Multi-wavelength light amplifier

ABSTRACT

A multi-wavelength light amplifier includes a first-stage light amplifier which has a first light amplifying optical fiber amplifying a light input, a second-stage light amplifier which has a second light amplifying optical fiber amplifying a first light output from the first-stage light amplifier, and an optical system which maintains a second light output of the second-stage light amplifier at a constant power level. The first-stage and second-stage light amplifiers have different gain vs wavelength characteristics so that the multi-wavelength light amplifier has no wavelength-dependence of a gain thereof.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention generally relates to a light amplifier fora wavelength division multiplexed (WDM) optical transmission system, andmore particularly to a light amplifier having a two-stage configurationwhich eliminates a wavelength-dependence of the gain of the lightamplifier.

[0003] Recently, an optical communications network has increasingly beenused in practice. Nowadays, it is required that the opticalcommunications network cope with multi-media networking. A WDM system ismore attractive, particularly in terms of an increase in thetransmission capacity. In order to realize the WDM system, it isnecessary to use a multi-wavelength light amplifier capable ofamplifying a wavelength division multiplexed signal. It is required thatsuch a multi-wavelength light amplifier does not havewavelength-dependence of the gain, which is further required not to bechanged due to a variation in the power of the input light.

[0004] A light amplifier is known which has an optical fiber doped witha rare-earth element and directly amplifies the input light. There hasbeen some activity in the development of a multi-wavelength lightamplifier which amplifiers a wavelength division multiplexed lightsignal including signal components having different wavelengths(channels).

[0005] However, normally, the rare-earth-element doped fiber amplifierhas a very narrow range in which the gain thereof does not have thewavelength-dependence. In this regard, nowadays, there is no availablelight amplifier which can practically be used for the WDM system. Thatis, there is no available light amplifier which does not havewavelength-dependence of the gain, which is not changed due to avariation in the power of the input light. Particularly, thewavelength-dependence of the gain, which takes place when the inputpower changes, deteriorates the signal-to-noise ratio with respect to aparticular signal. This prevents the multi-wavelength light amplifierfrom being used in practice.

SUMMARY OF THE INVENTION

[0006] It is a general object of the present invention to provide amulti-wavelength light amplifier in which the above disadvantages areeliminated.

[0007] A more specific object of the present invention is to provide amulti-wavelength light amplifier which does not havewavelength-dependence of the gain, which is not changed due to avariation in the power of the input light.

[0008] The above objects of the present invention are achieved by amulti-wavelength light amplifier comprising: a first-stage lightamplifier which has a first light amplifying optical fiber amplifying alight input; a second-stage light amplifier which has a second lightamplifying optical fiber amplifying a first light output from thefirst-stage light amplifier; and an optical system which maintains asecond light output of the second-stage light amplifier at a constantpower level. The first-stage and second-stage light amplifiers havedifferent gain vs wavelength characteristics so that themulti-wavelength light amplifier has no wavelength-dependence of a gain.

[0009] The above multi-wavelength light amplifier may be configured asfollows. The first-stage light amplifier comprises a first pump sourcewhich pumps the first light amplifying optical fiber so as to have afirst gain vs wavelength characteristic in which as a wavelength oflight to be amplified becomes shorter, a gain of the first-stage lightamplifier becomes higher. The second-stage light amplifier comprises asecond pump source which pumps the second light amplifying optical fiberso as to have a second gain vs wavelength characteristic in which as awavelength of light to be amplified becomes longer, a gain of thefirst-stage light amplifier becomes higher.

[0010] The above multi-wavelength light amplifier may be configured asfollows. The first-stage light amplifier comprises a first pump sourcewhich pumps the first light amplifying optical fiber so as to have afirst gain vs wavelength characteristic having a first linear gainslope. The second-stage light amplifier comprises a second pump sourcewhich pumps the second light amplifying optical fiber so as to have asecond gain vs wavelength characteristic having a second linear gainslope. A combination of the first and second linear gain slopes resultsin a flat gain vs wavelength characteristic of the multi-wavelengthlight amplifier.

[0011] The above multi-wavelength light amplifier may further comprisean optical filter which emphasizes the gain vs wavelength characteristicof the first-stage light amplifier.

[0012] The above multi-wavelength light amplifier may further comprisean optical filter which compensates for a difference between the gain vswavelength characteristics of the first-stage light amplifier and thesecond-stage light amplifier.

[0013] The above multi-wavelength light amplifier may be configured asfollows. The optical filter is provided so as to follow the first-stagelight amplifier. The first-stage light amplifier comprises a first pumpsource which pumps the first light amplifying optical fiber so as tohave a first gain vs wavelength characteristic having a first lineargain slope. The second-stage light amplifier comprises a second pumpsource which pumps the second light amplifying optical fiber so as tohave a second gain vs wavelength characteristic having a second lineargain slope. The optical filter emphasizes the first linear gain slope toprovide an emphasized first linear gain slope. A combination of theemphasized first linear slope and the second linear gain slope resultsin a flat gain vs wavelength characteristic of the multi-wavelengthlight amplifier.

[0014] The above multi-wavelength light amplifier may be configured asfollows. The optical filter is provided so as to follow the first-stagelight amplifier. The first-stage light amplifier comprises a first pumpsource which pumps the first light amplifying optical fiber so as tohave a first gain vs wavelength characteristic having a first lineargain slope. The second-stage light amplifier comprises a second pumpsource which pumps the second light amplifying optical fiber so as tohave a second gain vs wavelength characteristic having a second lineargain slope. The optical filter compensates for the difference betweenthe first and second linear gain slopes so that a flat gain vswavelength characteristic of the multi-wavelength light amplifier can beobtained.

[0015] The above multi-wavelength light amplifier may be configured asfollows. The first-stage light amplifier has a first AGC (automatic gaincontrol) system so that a ratio of the input light and the first lightoutput is constant. The second-stage light amplifier has a second AGCsystem so that a ratio of the first light output and the second lightoutput is constant.

[0016] The above multi-wavelength light amplifier may be configured asfollows. The first-stage light amplifier has an AGC (automatic gaincontrol) system so that a ratio of the input light and the first lightoutput is constant. The second-stage light amplifier has an automaticpower control (APC) system so that the second light amplifying opticalfiber is pumped at a predetermined constant power level.

[0017] The above multi-wavelength light amplifier may be configured asfollows. The first-stage light amplifier has an AGC (automatic gaincontrol) system so that a ratio of the input light and the first lightoutput is constant. The second-stage light amplifier has an automaticlevel control (ALC) system so that the second light output is maintainedat a predetermined constant power level.

[0018] The above multi-wavelength light amplifier may be configured asfollows. The first AGC system comprises first means for detecting afirst level of the light input and a second level of the first lightoutput and pumping the first light amplifying optical fiber so that aratio of the first and second levels is maintained at a firstpredetermined constant value. The second AGC system comprises secondmeans for detecting a third level of the first light output and a fourthlevel of the second light output and pumping the second light amplifyingoptical fiber so that a ratio of the third and fourth levels ismaintained at a second predetermined constant value.

[0019] The above multi-wavelength light amplifier may be configured asfollows. The first-stage light amplifier has a first AGC (automatic gaincontrol) system which detects a first amplified spontaneous emission ofthe first light amplifying optical fiber and pumps the first lightamplifying optical fiber so that the first amplified spontaneousemission is maintained at a first predetermined constant level. Thesecond-stage light amplifier has a second AGC system which detects asecond amplified spontaneous emission of the second light amplifyingoptical fiber and pumps the second light amplifying optical fiber sothat the second amplified spontaneous emission is maintained at a secondpredetermined constant level.

[0020] The above multi-wavelength light amplifier may be configured asfollows. The first-stage light amplifier has a first AGC (automatic gaincontrol) system which detects a first pump light propagated through thefirst light amplifying optical fiber and pumps the first lightamplifying optical fiber so that the first pump light is maintained at afirst predetermined constant level., The second-stage light amplifierhas a second AGC system which detects a second pump light propagatedthrough the second light amplifying optical fiber and pumps the secondlight amplifying optical fiber so that the second pump light ismaintained at a second predetermined constant level.

[0021] The above multi-wavelength light amplifier may be configured asfollows. The first-stage light amplifier comprises a first pump sourcewhich pumps the first light amplifying optical fiber through a firstcoupler so as to have a first gain vs wavelength characteristic in whichas a wavelength of light to be amplified becomes shorter, a gain of thefirst-stage light amplifier becomes higher. The second-stage lightamplifier comprises a second pump source which pumps the second lightamplifying optical fiber through a second coupler so as to have a secondgain vs wavelength characteristic in which as a wavelength of light tobe amplified becomes longer, a gain of the first-stage light amplifierbecomes higher. At least one of the first and second couplers has acharacteristic which emphasizes one of the gain vs wavelengthcharacteristics of the first-stage and second-stage light amplifiers.

[0022] The above multi-wavelength light amplifier may be configured asfollows. The optical system which maintains the second light output ofthe second-stage light amplifier at a constant power level comprises avariable attenuator which is provided between the first-stage lightamplifier and the second-stage light amplifier and attenuates the firstoutput signal on the basis of the power level of the second lightoutput.

[0023] The above multi-wavelength light amplifier may be configured asfollows. The optical system which maintains the second light output ofthe second-stage light amplifier at a constant power level comprises avariable attenuator which is provided so as to follow the second-stagelight amplifier and attenuates the second output signal on the basis ofthe power level of an attenuated second light output from the variableattenuator.

[0024] The above multi-wavelength light amplifier may be configured asfollows. The optical system which maintains the second light output ofthe second-stage light amplifier at a constant power level comprises avariable attenuator which is provided between the first-stage lightamplifier and the second-stage light amplifier and attenuates the firstoutput signal on the basis of the power level of an attenuated firstlight output from the variable attenuator.

[0025] The above multi-wavelength light amplifier may further comprise arejection filter which is provided between the first-stage lightamplifier and the second-stage light amplifier and prevents a pump lightwhich pumps the first light amplifying optical fiber from beingtransmitted to the second-stage light amplifier.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Other objects, features and advantages of the present inventionwill become more apparent from the following detailed description whenread in conjunction with the accompanying drawings, in which:

[0027]FIG. 1 is a block diagram of a multi-wavelength light amplifieraccording to a first embodiment of the present invention;

[0028]FIG. 2 is a-diagram showing a principle of a multi-wavelengthlight amplifier according to a second embodiment of the presentinvention;

[0029]FIG. 3 is a block diagram of a multi-wavelength light amplifieraccording to a third embodiment of the present invention;

[0030]FIG. 4 is a diagram showing a principle of the multi-wavelengthlight amplifier according to the third embodiment of the presentinvention;

[0031]FIG. 5 is a block diagram of a multi-wavelength light amplifieraccording to a fourth embodiment of the present invention;

[0032]FIG. 6 is a diagram showing a principle of the multi-wavelengthlight amplifier according to the fourth embodiment of the presentinvention;

[0033]FIGS. 7A and 7B are diagrams showing a principle of amulti-wavelength light amplifier according to a fifth embodiment of thepresent invention;

[0034]FIG. 8 is a block diagram of a multi-wavelength light amplifieraccording to a seventh embodiment of the present invention;

[0035]FIG. 9 is a block diagram of a multi-wavelength light amplifieraccording to an eighth embodiment of the present invention;

[0036]FIG. 10 is a block diagram of a multi-wavelength light amplifieraccording to a ninth embodiment of the present invention;

[0037]FIG. 11 is a block diagram of a multi-wavelength light amplifieraccording to a tenth embodiment of the present invention;

[0038]FIG. 12 is a block diagram of a multi-wavelength light amplifieraccording to an eleventh embodiment of the present invention;

[0039]FIG. 13 is a block diagram of a multi-wavelength light amplifieraccording to a twelfth embodiment of the present invention;

[0040]FIG. 14 is a block diagram of a multi-wavelength light amplifieraccording to a thirteenth embodiment of the present invention; and

[0041]FIG. 15 is a block diagram of a multi-wavelength light amplifieraccording to a fourteenth embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0042]FIG. 1 is a block diagram of a multi-wavelength light amplifieraccording to a first embodiment of the present invention. The amplifiershown in FIG. 1 includes a first-stage (front-stage) light amplifier 1and a second-stage (rear-stage) light amplifier 2. A variable attenuator(ATT) 11 is provided between the first and second amplifiers 1 and 2.The variable attenuator 11 is controlled by an automatic level control(ALC) circuit 14, which is controlled by a photodetector 13 such as aphotodiode. The photodiode 13 receives split light from a beam splittingcoupler 12, which follows the second-stage amplifier 2. An opticalsystem having a feedback loop is formed by the light splitting coupler12, the photodiode 13, the ALC circuit 14 and the variable attenuator11.

[0043] The first-stage amplifier 1 includes a first-stage light inputmonitor made up of a beam splitting coupler 3 ₁ and a photodiode 4 ₁,and a first-stage light output monitor made up of a beam splittingcoupler 3 ₂ and a photodiode 4 ₂. Further, the first-stage amplifier 1includes a light amplifying optical fiber 7 such as a rare-earth-elementdoped optical fiber and an exciting-light source (hereinafter referredto as a pump source: PS) 9 ₁ which is controlled by an automatic gaincontrol (AGC) circuit 6 ₁ provided in the first-stage amplifier 1. AnAGC system including the AGC circuit 6 ₁ and the above input and outputmonitors performs an AGC control of the pump source 9 ₁ so-that theratio of the light input power level detected by the light input monitorand the light output power level detected by the light output monitorcan be maintained at a constant value. The above ratio corresponds tothe gain of the first-stage amplifier 1.

[0044] The second-stage amplifier 2 includes a second-stage light inputmonitor made up of a beam splitting coupler 3 ₃ and a photodiode 4 ₃,and a second-stage light output monitor made up of a beam splittingcoupler 3 ₄ and a photodiode 4 ₄. Further, the second-stage amplifier 2includes a light amplifying optical fiber 8 such as rare-earth-elementdoped optical fiber, and a pump source 9 ₂, which is controlled by anAGC circuit 6 ₂ provided in the second-stage amplifier 2. An AGC systemincluding the AGC circuit 6 ₂ and the above input and output monitorsperforms a AGC operation of the pump source 9 ₂ so that the ratio of thelight input power level detected by the light input monitor and thelight output power level detected by the light output monitor can bemaintained at a constant value.

[0045] The combination of the first-stage amplifier 1 and thesecond-stage amplifier 2 functions to cancel the difference between thegain of the amplifier 1 and the gain of the amplifier 2 in each of thewavelengths of the multiplexed signal. That is, the amplifiers 1 and 2have different gain vs. wavelength characteristics (which may be simplyreferred to as gain characteristics), which can be compensated by thecombination of the amplifiers 1 and 2. As a result, the entiremulti-wavelength light amplifier has a flat gain vs wavelengthcharacteristic.

[0046] It will now be assumed that G_(0,1) denotes an AGC controlsetting level which causes the amplifier 1 to have a flat gain vswavelength characteristic in which the output spectra at the respectivewavelengths of the multiplexed signal have a constant peak value.Similarly, G_(0,2) is denoted as an AGC control setting level whichcauses the amplifier 2 to have a flat gain vs wavelength characteristicin which the output spectra at the respective wavelengths of themultiplexed signal have a constant peak value. In order to achieve theabove cancellation, the practical AGC control setting levels G₁ and G₂of the amplifiers 1 and 2 are set so that G₁≧G_(0,1) and G₂≦G_(0,2). Inthis case, as will be described later with reference to FIG. 2, theamplifiers 1 and 2 can have gain vs wavelength characteristics that canbe compensated by the combination thereof. For example, the gain of theamplifier 1 at a wavelength is large, while the gain of the amplifier 2at the same wavelength as described above is small. Hence, the totalgain obtained by the amplifiers 1 and 2 can be maintained at a constant(flat) level. By combining the two amplifiers together as describedabove, it is possible for the multi-wavelength light amplifier to haveno waveform-dependence of the gain thereof.

[0047] The above waveform-dependence of the gain can be maintained at aconstant level irrespective of a variation in the input power by meansof the feedback loop including the light splitting coupler 12, thephotodiode 13, the ALC circuit 14 and the variable attenuator 11. Thesplit light from the beam splitting coupler 12 is applied to thephotodiode 13, which generates an electric signal corresponding to thelight level. The above electric signal is applied to the variableattenuator 11, and the amount of attenuation caused therein is varied onthe basis of the light level detected by the photodiode 13. In thismanner, the light output level of the second-stage amplifier 2 can bemaintained at a constant level. The variable attenuator 11 may be formedby using a Faraday rotator or the electro-optical effect of a lithiumniobate (LiNbO₃) crystal.

[0048] The amplifiers 1 and 2 are pumped forward by the pump sources 9 ₁and 9 ₂. Alternatively, it is possible to pump the amplifiers 1 and 2backward. It is also possible to pump the amplifiers 1 and 2 forward andbackward.

[0049] The light amplifier shown in FIG. 1 is capable of amplifying allthe wavelengths to be multiplexed so that the light amplifier does nothave the wavelength-dependence of the gain, which is not changed due toa variation in the power of the input light. If some wavelengths are notused or only some wavelengths are used, a filter (not shown) having acorresponding wavelength characteristic may be placed before thephotodiode 4 ₁ (4 ₃) or 4 ₂ (4 ₄) of the first-stage (second-stage)amplifier 1 (2) or both thereof.

[0050]FIG. 2 is a diagram of the operation of a multi-wavelength lightamplifier according to a second embodiment of the present invention. Thesecond embodiment has the same configuration as shown in FIG. 1.According to the second embodiment of the present invention, the opticalfibers 7 and 8 are erbium-doped (Er-doped) optical fibers, which areexamples of rare-earth-element doped optical fibers. Normally, alumina(Al₂O₃) is added to the Er-doped optical fibers at a high concentrationlevel. In this regard, the Er-doped optical fiber may be called aco-doped optical fiber. The Er-doped optical fiber has a substantiallylinear gain vs wavelength characteristic in an amplifying band about1550 nm, as shown in FIG. 2.

[0051] Part (a) of FIG. 2 shows a gain vs wavelength characteristicobtained in the amplifying band about 1550 nm when the exciting rate isrelatively high, and part (b) of FIG. 2 shows a gain vs wavelengthcharacteristic obtained in the amplifying band about 1550 nm when theexciting rate is relative low. The characteristics shown in parts (a)and (b) of FIG. 2 are due to the characteristics of absorption/emissionof Er ions in the Er-doped optical fiber with alumina added thereto at ahigh concentration level. The horizontal axes of the parts (a), (b) and(c) of FIG. 2 denote the wavelength, and the vertical axes thereofdenote the gain of the Er-doped optical fiber.

[0052] As shown in part (a) of FIG. 2, in the amplifying band about 1550nm, the fiber has a relatively high gain on the short-wavelength side,and a relatively low gain on the long-wavelength side. In other words,as the wavelength becomes shorter, the gain becomes higher. As shown inpart (b) of FIG. 2, in the amplifying band about 1550 nm, the fiber hasa relatively high gain on the long-wavelength side, and a relatively lowgain on the short-wavelength side. In other words, as the wavelengthbecomes longer, the gain becomes higher.

[0053] According to the second embodiment of the present invention, theEr-doped fiber 7 of the first amplifier 1 is long enough to increase theexciting rate and obtain the characteristic shown in part (a) of FIG. 2.The Er-doped fiber 8 of the second amplifier 1 is short enough todecrease the exciting rate and obtain the characteristic shown in part(b) of FIG. 2. Generally, when the pumping of the Er-doped fiber isincreased, the gain vs wavelength characteristic is changed from part(b) of FIG. 2 to part (a) through part (c).

[0054] The linear gain slope characteristic of the first-stage amplifier1 and that of the gain characteristic of the second-stage amplifier 2are canceled by the combination of the amplifiers 1 and 2, so that aflat gain vs wavelength characteristic (a spectrum characteristic havinga constant gain) as shown in part (c) of FIG. 2 can be obtained.

[0055] It is preferable for the first-stage amplifier 1 to be a lownoise figure. In this regard, the Er-doped fiber 7 of the first-stageamplifier is used at a relatively high exciting rate. In this case, theexciting efficiency is not high. The Er-doped fiber 8 is used at arelatively low exciting rate. Hence, it is possible to improve theexciting efficiency of the second-stage amplifier 1. This contributes toreducing energy consumed in the second-stage amplifier 2.

[0056] The following data has been obtained through an experiment inwhich the multi-wavelength light amplifier was actually produced. Thelight amplifier produced in the experiment was designed to amplify fourwavelengths (1548 nm, 1551 nm, 1554 nm, 1557 nm). The light input levelused in the experiment was selected so as to fall within the range of−25 dBm through −15 dBm. The gain and the gain tilt of the first-stageamplifier 1 were respectively set to 20 dB and 1.5 dB at a maximum powerof the exciting light equal to −160 mW (980 nm). The second-stageamplifier 2 was adjusted so as to produce, for each channel, the lightoutput equal to +7 dBm at a maximum power of the exciting light equal to−100 mW (1480 nm). In this case, the multi-wavelength light amplifierhas a maximum noise figure of 5.6 dB and a maximum gain tilt of 0.2 dB.

[0057]FIG. 3 is a block diagram of a multi-wavelength light amplifieraccording to a third embodiment of the present invention. In FIG. 3,parts that are the same as those shown in FIG. 1 are indicated by thesame reference numbers. The light amplifier shown in FIG. 3 has anoptical filter 15 for compensating for a wavelength characteristic, aswill be described below. The optical filter 15 is provided between thevariable attenuator 11 and the input side of the second-stage amplifier2.

[0058]FIG. 4 is a diagram showing the operation of the light amplifiershown in FIG. 3. More particularly, part (a) of FIG. 4 shows a gain vswavelength characteristic of the first-stage amplifier 1 shown in FIG.3, and part (b) thereof shows a gain vs wavelength characteristicobtained by the combination of the first-stage amplifier 1 and theoptical filter 15. Part (c) of FIG. 4 shows a gain vs wavelengthcharacteristic of the second-stage amplifier 2 shown in FIG. 4, and part(d) shows a total gain vs wavelength characteristic of the whole lightamplifier shown in FIG. 3.

[0059] The configuration of the first-stage amplifier 1 shown in FIG. 3is the same as that of the amplifier 1 shown in FIG. 1. Theconfiguration of the second-stage amplifier 2 shown in FIG. 3 is thesame as that of the amplifier 2 shown in FIG. 1.

[0060] The optical filter 15 emphasizes the gain vs wavelengthcharacteristic of the first-stage amplifier 1. As shown in parts (a) and(b) of FIG. 4, the gain for the short wavelengths is particularlyemphasized. In other words, the linear gain slope of the characteristicshown in part (a) of FIG. 4 is increased by the optical filter 15. Thecharacteristic of the second-stage amplifier 2 shown in part (c) of FIG.4 compensates for the characteristic shown in part (b) thereof, so thatthe flat gain characteristic shown in part (d) of FIG. 4 can be finallyobtained.

[0061] It will be noted that the exciting rate necessary to obtain thecharacteristic shown in part (c) of FIG. 4 is lower than that necessaryto obtain the characteristic shown in part (b) of FIG. 2. In otherwords, the exciting efficiency of the characteristic shown in part (c)of FIG. 4 is higher than that of the characteristic shown in part (b) ofFIG. 2. Hence, the second-stage amplifier 2 shown in FIG. 3 consumes asmaller, amount of energy than that shown in FIG. 1. In other words, ifthe second-stage amplifier 2 shown in FIG. 3 consumes the same amount ofenergy as that shown in FIG. 1, the multi-wavelength light amplifiershown in FIG. 3 can output a larger amount of power than that shown inFIG. 1.

[0062] Since the first-stage amplifier 1 has the characteristic shown inpart (a) of FIG. 4, it is a low noise figure. The characteristic of thefirst-stage amplifier 1 is emphasized by the optical filter 15, and theexciting efficiency thereof may be improved.

[0063] The variable attenuator 11 shown in FIG. 3 is controlled in thesame manner as that shown in FIG. 1 as has been described previously. Inshort, the variable attenuator 11 maintains the level of the outputlight of the second-stage amplifier 1 at the predetermined constantlevel.

[0064]FIG. 5 is a block diagram of a multi-wavelength light amplifieraccording to a fourth embodiment of the present invention. In FIG. 5,parts that are the same as those shown in the previously describedfigures are given the same reference numbers. The configuration shown inFIG. 5 differs from that shown in FIG. 3 in that the optical filter 15shown in FIG. 5 is provided between the output side of the second-stageamplifier 2 and the beam splitting coupler 12.

[0065]FIG. 6 is a diagram showing the operation of the light amplifiershown in FIG. 5. More particularly, part (a) of FIG. 6 shows a gain vswavelength characteristic of the first-stage amplifier 1 shown in FIG.5, and part (b) thereof shows a gain vs wavelength characteristic of thesecond-stage amplifier 2 shown in FIG. 5. Part (c) of FIG. 5 is a gainvs-wavelength characteristic obtained by the combination of thefirst-stage amplifier 1 and the second-stage amplifier 2. Part (d) ofFIG. 6 shows a total gain vs wavelength characteristic of the wholelight amplifier shown in FIG. 5.

[0066] The configuration of the first-stage amplifier 1 shown in FIG. 5is the same as that of the amplifier 1 shown in FIGS. 1 and 3. Theconfiguration of the second-stage amplifier 2 shown in FIG. 5 is thesame as that of the amplifier 2 shown in FIGS. 1 and 3.

[0067] The optical filter 15 has a gain vs wavelength characteristicwhich compensates for that shown in part (b) of FIG. 2. As shown inparts (a) and (b) of FIG. 6, the characteristic of the second-stageamplifier 2 is pumped so as to have an emphasized gain vs wavelengthcharacteristic, as compared to that of the first-stage amplifier 1. Inthe emphasized characteristic, the gain for the long wavelengths isparticularly emphasized. In other words, the linear gain slope of thecharacteristic shown in part (b) of FIG. 6 is greater than that shown inpart (a) thereof although the linear gain slopes shown in parts (a) and(b) thereof are oriented in different directions. The combination of thefirst-stage amplifier 1 and the second-stage amplifier 2 results in thecharacteristic shown in part (c) of FIG. 6. It is not required that thefirst-stage amplifier 1 and the second-stage amplifier 2 havecharacteristics of such a difference which can be completely canceled bythe combination thereof.

[0068] The optical filter 15 shown in FIG. 5 has a gain vs wavelengthcharacteristic which compensates for the characteristic shown in part(c) of FIG. 6. Thus, the total characteristic is as shown in part (d) ofFIG. 6.

[0069] It will be noted that the exciting rate necessary to obtain thecharacteristic shown in part (b) of FIG. 6 is lower than that necessaryto obtain the characteristic shown in part (b) of FIG. 2. In otherwords, the exciting efficiency of the characteristic shown in part (b)of FIG. 6 is higher than that of the characteristic shown in part (b) ofFIG. 2. Hence, the second-stage amplifier 2 shown in FIG. 5 consumes asmaller amount of energy than that shown in FIG. 1. In other words, ifthe second-stage amplifier 2 shown in FIG. 5 consumes the same amount ofenergy as that shown in FIG. 1, the multi-wavelength light amplifiershown in FIG. 5 can output a larger amount of power than that shown inFIG. 1.

[0070] The variable attenuator 11 shown in FIG. 5 is controlled in thesame manner as that shown in FIG. 1 as has been described previously. Inshort, the variable attenuator 11 shown in FIG. 5 maintains the level ofthe output light of the second-stage amplifier 1 at the predeterminedconstant level.

[0071] The optical filter 15 used in FIG. 3 or FIG. 5 may be aconventional coupler of a melting attachment type. By adjusting thewavelength period of the coupler, it is possible to use the coupler as again tilting filter. For example, the optical filter 15 shown in FIG. 5has a gain tilt equal to approximately 3 dB in order to obtain the flatgain characteristic shown in part (d) of FIG. 6.

[0072] A description will now be given of a multi-wavelength lightamplifier according to a fifth embodiment of the present invention. Thisembodiment is intended to obtain the same function as the configurationshown in FIG. 3 without the optical filter 15 shown therein. In otherwords, the light amplifier according to the fifth embodiment isconfigured as shown in FIG. 1, nevertheless it has the function of thelight amplifier shown in FIG. 3.

[0073] According to the fifth embodiment of the present invention, thebeam splitting coupler 5 ₂ is replaced by a beam splitting coupler 21shown in FIG. 7A, which has a transparent rate vs wavelengthcharacteristic as shown in FIG. 7B. In FIG. 7A, a pump source 22 whichcorresponds to the pump source 9 ₂ is coupled to the beam splittingcoupler 21. In FIG. 7B, symbol λ_(p) denotes the wavelength of the pumplight emitted from the source 22. Symbol λ_(s) denotes the centralwavelength of the multiplexed light signal. Symbols λ_(sl) and λ_(sn)are wavelengths which define the band of the multiplexed light signal. Asolid line shown in FIG. 7B denotes a characteristic used forcommunications. Two dot lines are obtained by shifting the solid line.As indicated by the solid line, the beam splitting coupler 21 functionsto pass the multiplexed signal light and prevent the pump light in theforward direction.

[0074] By shifting the solid line toward the short-wavelength side asindicated by character A in FIG. 7B, the characteristic curve of thetransparent rate has a slope in the band defined by the wavelengths λsland λ_(sn). In this case, the highest transparent rate can be obtainedat the shortest wavelength λ_(sl), and the lowest transparent rate canbe obtained at the longest wavelength λ_(sn). This characteristiccorresponds to the characteristic of the optical filter 15 used in theconfiguration shown in FIG. 3. With the above configuration, themulti-wavelength light amplifier according to the fifth embodiment ofthe present invention has the same advantages as those of the lightamplifier shown in FIG. 3.

[0075] The beam splitting coupler 21 can be applied to the first-stageamplifier 1 instead of the second-stage amplifier 2. In this case, theEr-doped optical fiber 7 of the first-stage amplifier 1 is pumpedbackward by the pump source 22 because the optical filter 15 shown inFIG. 3 is placed on the output side of the Er-doped optical fiber 7.

[0076] A description will now be given of a multi-wavelength lightamplifier according to a sixth embodiment of the present invention. Thisembodiment is intended to obtain the same function as the configurationshown in FIG. 5 without the optical filter 15 shown therein. In otherwords, the light amplifier according to the sixth embodiment isconfigured as shown in FIG. 1, nevertheless it has the function of thelight amplifier shown in FIG. 5.

[0077] In the sixth embodiment of the present invention, the pump source9 ₂ shown in FIG. 1 is replaced by the pump source 22 shown in FIG. 7Ahaving the transparent rate characteristic indicated by B shown in FIG.7B in such a way that the Er-doped optical fiber 8 is pumped backward bythe pump source 22. This is because the optical filter 15 shown in FIG.5 is placed on the output side of the Er-doped optical fiber 8 shown inFIG. 5.

[0078] By shifting the solid line shown in FIG. 7B toward thelong-wavelength side as indicated by character B, the characteristiccurve of the transparent rate has a slope in the band defined by thewavelengths λ_(sl) and λ_(sn). In this case, the highest transparentrate can be obtained at the longest wavelength λ_(sn), and the lowesttransparent rate can be obtained at the shortest wavelength λ_(sl). Thischaracteristic corresponds to the characteristic of the optical filter15 used in the configuration shown in FIG. 5. With the aboveconfiguration, the multi-wavelength light amplifier according to thesixth embodiment of the present invention has the same advantages asthose of the light amplifier shown in FIG. 5.

[0079] It will be noted that the above-mentioned third through sixthembodiments of the present invention may be combined appropriately.

[0080]FIG. 8 is a multi-wavelength light amplifier according to aseventh embodiment of the pre sent invention. In FIG. 8, parts that arethe same as those shown in the previously described figures are giventhe same reference numbers. The light amplifier shown in FIG. 8 has asecond-stage light amplifier 2A having a configuration different fromthe above-mentioned second-stage light amplifier 2.

[0081] More particularly, the second-stage amplifier 2A has an automaticpower control (APC) circuit 10. The APC circuit 10 monitors and controlsthe pump light emitted from the pump source 9 ₂, so that the pump lightcan be emitted at a predetermined constant level. As has been describedpreviously, the variable attenuator 11 functions to maintain theamplified light output by the second-stage amplifier 2 at thepredetermined constant level. Hence, even by the automatic power controlof the pump light directed to maintaining the pump light at the constantlevel, it is possible to maintain the output light of the second-stageamplifier 2A at the predetermined constant level even if the power ofthe light input signal fluctuates.

[0082] The first-stage amplifier 1 shown in FIG. 8 has a gain vswavelength characteristic as shown in part (a) of FIG. 2, and thesecond-stage amplifier 2A shown in FIG. 8 has a gain vs wavelengthcharacteristic as shown in part (b) of FIG. 2.

[0083] The second-stage amplifier 2A does not need the couplers 3 ₃ and3 ₄, and the photodiodes 4 ₃ and 4 ₄. Hence, the second-stage amplifier2A is simpler than the second-stage amplifier 2, so that down-sizing ofthe light amplifier can be facilitated.

[0084]FIG. 9 is a block diagram of a multi-wavelength light amplifieraccording to an eighth embodiment of the present invention. In FIG. 9,parts that are the same as those shown in the previously describedfigures are given the same reference numbers. The configuration shown inFIG. 9 differs from the configuration shown in FIG. 1 in that thevariable attenuator 11 shown in FIG. 9 is provided on the output side ofthe second-stage amplifier 2. Thus, the variable attenuator 11attenuates the output light signal of the second-stage amplifier 2 sothat it can be maintained at the predetermined constant level.

[0085] It will be noted that in the configuration shown in FIG. 1, theattenuated light signal from the variable attenuator 11 is amplified bythe second-stage amplifier 2. On the other hand, in the configurationshown in FIG. 9, the variable attenuator 11 attenuates the light outputsignal of the second-stage amplifier 2. Hence, the second-stageamplifier 2 shown in FIG. 9 needs a much larger amount of energy of thepump light than that used in the configuration shown in FIG. 1. However,except for the above, the light amplifier shown in FIG. 9 has the sameadvantages as the configuration shown in FIG. 1. For example, the lightamplifier shown in FIG. 9 has a low noise figure because an increase inloss of the gain does not occur between the first-stage amplifier 1 andthe second-stage amplifier 2.

[0086] It will be noted that the first-stage and second-stage amplifiers1 and 2 (2A) are not limited to the previously described AGC (APC)circuits in order to obtain the characteristics shown in FIGS. 2, 4 and6. It is possible to arbitrarily combine the previously described AGCcircuits. Further, it is also possible to employ other AGC circuits orequivalents thereof, which will be described below as ninth througheleventh embodiments of the present invention. It will be noted that theAGC circuit of the first-stage circuit can be selected separately fromthe AGC circuit of the second-stage circuit.

[0087]FIG. 10 is a block diagram of a multi-wavelength light amplifieraccording to a ninth embodiment of the present invention, wherein partsthat are the same as those shown in FIG. 1 are given the same referencenumbers. The light amplifier shown in FIG. 10 has a first-stageamplifier 1B and a second-stage amplifier 2B, which are different fromthe amplifiers 1 and 2.

[0088] The first-stage amplifier 1B, which has a gain vs wavelengthcharacteristic as shown in part (a) of FIG. 2, has a forward-directionphotodiode 201, which detects an amplified spontaneous emission (ASE)leaking from the side surface of the Er-doped optical fiber 7. The AGCcircuit 6 ₁ is supplied with the output signal of the photodiode 20 ₁and controls the pump power of the pump source 9 ₁ so that the amplifiedspontaneous emission can be maintained at a predetermined constantlevel. As a result of the AGC control, the gain of the front-stageamplifier 1B can be maintained at the predetermined constant value.

[0089] Similarly, the second-stage amplifier 2B, which has a gain vswavelength characteristic as shown in part (b) of FIG. 2, has aforward-direction photodiode 20 ₂, which detects the amplifiedspontaneous emission leaking from a side surface of the Er-doped opticalfiber 8. The AGC circuit 6 ₂ is supplied with the output signal of thephotodiode 20 ₂ and controls the pump power of the pump source 9 ₂ sothat the amplified spontaneous emission can be maintained at apredetermined constant level. As a result of the above AGC control, thegain of the second-stage amplifier 2B can be maintained at thepredetermined constant level.

[0090] As has been described previously, the variable attenuator 11provided between the first-stage amplifier 1B and the second-stageamplifier 2B functions to maintain the light output level at thepredetermined constant level.

[0091]FIG. 11 is a block diagram of a multi-wavelength light amplifieraccording to a tenth embodiment of the present invention, in which partsthat are the same as those shown in the previously described figures aregiven the same reference numbers. The light amplifier shown in FIG. 11includes a first-stage light amplifier 1C and a second-stage lightamplifier 2C.

[0092] The first-stage light amplifier 1C, which has a gain vswavelength characteristic as shown in part (a) of FIG. 2, includes a WDMcoupler 16 ₁ and a photodiode 17 ₁. The WDM coupler 16 ₁ separates thelight in the 1530 nm band (ASE) from the light in the 1550 nm band(signal light). The above ASE travels toward the input side of theEr-doped optical fiber 7 (backward ASE). The photodiode 17 ₁ detects theamplified spontaneous emission of the Er-doped optical fiber 7. The AGCcircuit 6 ₁ receives the output signal of the photodiode 17 ₁ andcontrols the pump power of the pump source 9 ₁ so that the backward ASEcan be maintained at a predetermined constant level. As a result of theabove AGC control, the gain of the first-stage amplifier 1C can bemaintained at the predetermined constant level.

[0093] The second-stage light amplifier 2C, which has a gain vswavelength characteristic as shown in part (b) of FIG. 2, includes a WDMcoupler 16 ₂ and a photodiode 17 ₂. The WDM coupler 16 ₁ separates thelight in the 1530 nm band (ASE) from the light in the 1550 nm band(signal light). The above ASE travels toward the input side of theEr-doped optical fiber 8 (backward ASE). The photodiode 17 ₂ detects theamplified spontaneous emission of the Er-doped optical fiber 8. The AGCcircuit 6 ₂ receives the output signal of the photodiode 17 ₂ andcontrols the pump power of the pump source 9 ₂ so that the backward ASEcan be maintained at a predetermined constant level. As a result of theabove AGC control, the gain of the second-stage amplifier 2C can bemaintained at the predetermined constant level.

[0094] As has been described previously, the variable attenuator 11provided between the first-stage amplifier IC and the second-stageamplifier 2C functions to maintain the light output level at thepredetermined constant level.

[0095]FIG. 12 is a block diagram of a multi-wavelength light amplifieraccording to an eleventh embodiment of the present invention, in whichparts that are the same as those shown in the previously describedfigures are given the same reference numbers. The light amplifier shownin FIG. 12 includes a first-stage light amplifier ID and a second-stagelight amplifier 2D.

[0096] The first-stage light amplifier ID, which has a gain vswavelength characteristic as shown in part (a) of FIG. 2, includes a WDMcoupler 53 and a photodiode 181. The WDM coupler 53 is provided on theoutput side of the Er-doped optical fiber 7, and separates the residualpump light (exciting light) propagated through the fiber 7 from thesignal light. The residual pump light separated by the WDM coupler 5 ₃is applied to the photodiode 18 ₁, which outputs a correspondingelectric signal to the AGC circuit 6 ₁. Then, the AGC circuit 6 ₁controls the pump power of the pump source 9 ₁ on the basis of thedetected residual pump light so that the residual pump light can bemaintained at a predetermined constant level. As a result of the aboveAGC control, the gain of the first-stage amplifier ID can be maintainedat the predetermined constant level.

[0097] The second-stage light amplifier 2D, which has a gain vswavelength characteristic as shown in part (b) of FIG. 2, includes a WDMcoupler 5 ₄ and a photodiode 18 ₂. The WDM coupler 5 ₄ is provided onthe output side of the Er-doped optical fiber 8, and separates theresidual pump light (exciting light) propagated through the fiber 8 fromthe signal light. The residual pump light separated by the WDM coupler 5₄ is applied to the photodiode 18 ₂, which outputs a correspondingelectric signal to the AGC circuit 6 ₂. Then, the AGC circuit 6 ₂controls the pump power of the pump source 9 ₂ on the basis of thedetected residual pump light so that the residual pump light can bemaintained at a predetermined constant level. As a result of the aboveAGC control, the gain of the second-stage amplifier 2D can be maintainedat the predetermined constant level.

[0098] As has been described previously, the variable attenuator 11provided between the first-stage amplifier 1D and the second-stageamplifier 2D functions to maintain the light output level at thepredetermined constant level.

[0099]FIG. 13 is a block diagram of a multi-wavelength light amplifieraccording to a twelfth embodiment of the present invention, whereinparts that are the same as those shown in the previously describedfigures are given the same reference numbers. The light amplifier shownin FIG. 13 differs from that shown in FIG. 1 in that the beam splittingcoupler 12 is provided between the variable attenuator 11 and thesecond-stage amplifier 2.

[0100] It is possible to maintain the light output of the second-stageamplifier 2 at the predetermined constant level by controlling thevariable attenuator 11 on the basis of the attenuated light output sothat the attenuated light output is maintained at a predeterminedconstant level. In order to realize the above feedback control, thephotodiode 13 detects a split component of the attenuated light output,and the ALC circuit 14 controls the variable attenuator 11 in theabove-described manner.

[0101]FIG. 14 is a block diagram of a multi-wavelength light amplifieraccording to a thirteenth embodiment of the present invention, in whichparts that are the same as those shown in the previously describedfigures are given the same reference numbers. The light amplifier shownin FIG. 14 corresponds to a modification of the light amplifier shown inFIG. 13. The light amplifier shown in FIG. 14 has the first-stage lightamplifier 1 and a second-stage light amplifier 2E.

[0102] The second-stage light amplifier 2E, which has a gain vswavelength characteristic as shown in part (b) of FIG. 2, includes abeam splitting coupler 3 ₄, the photodiode 4 ₄ and an ALC circuit 14 ₂.It will be noted that the second-stage amplifier 2E is simpler than thesecond-stage amplifier 2 shown in FIG. 13. As has been describedpreviously with reference to FIG. 13, the attenuated light output ismaintained at the predetermined constant level. Hence, the operation ofthe second-stage amplifier 2E receiving the attenuated light outputthrough the beam splitting coupler 12 is equivalent to theAGC-controlled operation of the second-stage amplifier. Hence, it ispossible to control the pump power of the pump source 9 ₂ by theautomatic level control performed by the ALC circuit 14 ₂.

[0103]FIG. 15 shows a multi-wavelength light amplifier according to afourteenth embodiment of the present invention. This amplifier includesa rejection filter 30 provided between the first-stage amplifier 1 andthe second-stage amplifier 2. The rejection filter 30 prevents the pumplight propagated from the Er-doped optical fiber 7 from passingtherethrough, and improves the exciting efficiency of the second-stageamplifier 2. The rejection filter 30 can be applied to the otherembodiments of the present invention in the same manner as shown in FIG.15.

[0104] The above-described embodiments of the present invention can bearbitrarily combined to provide variations and modifications.

[0105] The present invention is not limited to the specificallydisclosed embodiments, and variations and modifications may be madewithout departing from the scope of the present invention.

What is claimed is:
 1. A multi-wavelength light amplifier comprising: afirst-stage light amplifier which has a first light amplifying opticalfiber amplifying a light input; a second-stage light amplifier which hasa second light amplifying optical fiber amplifying a first light outputfrom the first-stage light amplifier; and an optical system whichmaintains a second light output of the second-stage light amplifier at aconstant power level, the first-stage and second-stage light amplifiershaving different gain vs wavelength characteristics so that themulti-wavelength light amplifier has no wavelength-dependence of a gainthereof.
 2. The multi-wavelength light amplifier as claimed in claim 1 ,wherein: the first-stage light amplifier comprises a first pump sourcewhich pumps the first light amplifying optical fiber so as to have afirst gain vs wavelength characteristic in which as a wavelength oflight to be amplified becomes shorter, a gain of the first-stage lightamplifier becomes higher; and the second-stage light amplifier comprisesa second pump source which pumps the second light amplifying opticalfiber so as to have a second gain vs wavelength characteristic in whichas a wavelength of light to be amplified becomes longer, a gain of thefirst-stage light amplifier becomes higher.
 3. The multi-wavelengthlight amplifier as claimed in claim 1 , wherein: the first-stage lightamplifier comprises a first pump source which pumps the first lightamplifying optical fiber so as to have a first gain vs wavelengthcharacteristic having a first linear gain slope; the second-stage lightamplifier comprises a second pump source which pumps the second lightamplifying optical fiber so as to have a second gain vs wavelengthcharacteristic having a second linear gain slope; and a combination ofthe first and second linear gain slopes provides a flat gain vswavelength characteristic of the multi-wavelength light amplifier. 4.The multi-wavelength light amplifier as claimed in claim 1 , furthercomprising an optical filter which emphasizes the gain vs wavelengthcharacteristic of the first-stage light amplifier.
 5. Themulti-wavelength light amplifier as claimed in claim 1 , furthercomprising an optical filter which compensates for a difference betweenthe gain vs wavelength characteristics of the first-stage lightamplifier and the second-stage light amplifier.
 6. The multi-wavelengthlight amplifier as claimed in claim 4 , wherein: the optical filter isprovided to follow the first-stage light amplifier; the first-stagelight amplifier comprises a first pump source which pumps the firstlight amplifying optical fiber so as to have a first gain vs wavelengthcharacteristic having a first linear gain slope; the second-stage lightamplifier comprises a second pump source which pumps the second lightamplifying optical fiber so as to have a second gain vs wavelengthcharacteristic having a second linear gain slope; the optical filteremphasizes the first linear gain slope to provide an emphasized firstlinear gain slope; a combination of the emphasized first linear slopeand the second linear gain slope results in a flat gain vs wavelengthcharacteristic of the multi-wavelength light amplifier.
 7. Themulti-wavelength light amplifier as claimed in claim 5 , wherein: theoptical filter is provided to follow the first-stage light amplifier;the first-stage light amplifier comprises a first pump source whichpumps the first light amplifying optical fiber so as to have a firstgain vs wavelength characteristic having a first linear gain slope; thesecond-stage light amplifier comprises a second pump source which pumpsthe second light amplifying optical fiber so as to have a second gain vswavelength characteristic having a second linear gain slope; the opticalfilter compensates for the difference between the first and secondlinear gain slopes so that a flat gain vs wavelength characteristic ofthe multi-wavelength light amplifier can be obtained.
 8. Themulti-wavelength light amplifier as claimed in claim 1 , wherein: thefirst-stage light amplifier has a first AGC (automatic gain control)system so that a ratio of the input light and the first light output isconstant; and the second-stage light amplifier has a second AGC systemso that a ratio of the first light output and the second light output isconstant.
 9. The multi-wavelength light amplifier as claimed in claim 1, wherein: the first-stage light amplifier has an AGC (automatic gaincontrol) system so that a ratio of the input light and the first lightoutput is constant; and the second-stage light amplifier has anautomatic power control (APC) system so that the second light amplifyingoptical fiber is pumped-at a predetermined constant power level.
 10. Themulti-wavelength light amplifier as claimed in claim 1 , wherein: thefirst-stage light amplifier has an AGC (automatic gain control) systemso that a ratio of the input light and the first light output isconstant; and the second-stage light amplifier has an automatic levelcontrol (ALC) system so that the second light output is maintained at apredetermined constant power level.
 11. The multi-wavelength lightamplifier as claimed in claim 8 , wherein: the first AGC systemcomprises first means for detecting a first level of the light input anda second level of the first light output and pumps the first lightamplifying optical fiber so that a ratio of the first and second levelsis maintained at a first predetermined constant value; and the secondAGC system comprises second means for detecting a third level of thefirst light output and a fourth level of the second light output andpumps the second light amplifying optical fiber so that a ratio of thethird and fourth levels is maintained at a second predetermined constantvalue.
 12. The multi-wavelength light amplifier as claimed in claim 1 ,wherein: the first-stage light amplifier has a first AGC (automatic gaincontrol) system which detects a first amplified spontaneous emission ofthe first light amplifying optical fiber and pumps the first lightamplifying optical fiber so that the first amplified spontaneousemission is maintained at a first predetermined constant level; and thesecond-stage light amplifier has a second AGC system which detects asecond amplified spontaneous emission of the second light amplifyingoptical fiber and pumps the second light amplifying optical fiber sothat the second amplified spontaneous emission is maintained at a secondpredetermined constant level.
 13. The multi-wavelength light amplifieras claimed in claim 1 , wherein: the first-stage light amplifier has afirst AGC (automatic gain control) system which detects a first pumplight propagated through the first light amplifying optical fiber andpumps the first light amplifying optical fiber so that the first pumplight is maintained at a first predetermined constant level; and thesecond-stage light amplifier has a second AGC system which detects asecond pump light propagated through the second light amplifying opticalfiber and pumps the second light amplifying optical fiber so that thesecond pump light is maintained at a second predetermined constantlevel.
 14. The multi-wavelength light amplifier as claimed in claim 1 ,wherein: the first-stage light amplifier comprises a first pump sourcewhich pumps the first light amplifying optical fiber through a firstcoupler so as to have a first gain vs wavelength characteristic in whichas a wavelength of light to be amplified becomes shorter, a gain of thefirst-stage light amplifier becomes higher; the second-stage lightamplifier comprises a second pump source which pumps the second lightamplifying optical fiber through a second coupler so as to have a secondgain vs wavelength characteristic in which as a wavelength of light tobe amplified becomes longer, a gain of the first-stage light amplifierbecomes higher; and at least one of the first and second couplers has acharacteristic which emphasizes one of the gain vs wavelengthcharacteristics of the first-stage and second-stage light amplifiers.15. The multi-wavelength light amplifier as claimed in claim 1 , furthercomprising a rejection filter provided between the first-stage lightamplifier and the second-stage light amplifier and prevents a pump lightwhich pumps the first light amplifying optical fiber from beingtransmitted to the second-stage light amplifier.
 16. Themulti-wavelength light amplifier as claimed in any of claims 1 to 15 ,wherein said optical system which maintains the second light output ofthe second-stage light amplifier at a constant power level comprises avariable attenuator provided between the first-stage light amplifier andthe second-stage light amplifier and attenuates the first output signalon the basis of the power level of the second light output.
 17. Themulti-wavelength light amplifier as claimed in any of claims 1 to 15 ,wherein said optical system which maintains the second light output ofthe second-stage light amplifier at a constant power level comprises avariable attenuator provided so as to follow the second-stage lightamplifier and attenuates the second output signal on the basis of thepower level of an attenuated second light output from the variableattenuator.
 18. The multi-wavelength light amplifier as claimed in anyof claims 1 to 15 , wherein said optical system which maintains thesecond light output of the second-stage light amplifier at a constantpower level comprises a variable attenuator provided between thefirst-stage light amplifier and the second-stage light amplifier andattenuates the first output signal on the basis of the power level of anattenuated first light output from the variable attenuator.
 19. Themulti-wavelength light amplifier as claimed in claim 1 , wherein thefirst and second light amplifying optical fibers are optical fibersdoped with a rare-earth element.
 20. The multi-wavelength lightamplifier as claimed in claim 19 , wherein the optical fibers doped withthe rare-earth element are erbium-doped optical fibers.